Near-Earth asteroid 1997 XF11, which briefly scared astronomers five years
ago, passes by Earth again this week on Halloween.

Five years ago astronomers had a fright.

Jim Scotti, using the 36-inch Spacewatch telescope on Kitt Peak, spotted a
dim speck of light moving through the constellation Cancer. It was an
asteroid–dark, about 1 km wide, and it seemed to be heading for Earth. The
Minor Planet Center named it “1997 XF11.”

Newspapers and magazines relayed the worst: 1997 XF11 might hit our planet
on Oct. 26, 2028. The impact, unleashing perhaps 2000 times more energy than
the most powerful nuclear weapon ever tested, would be a global catastrophe.

There is, however, an old amusement park proverb, “Fear – Death = Fun.” So
it proved for 1997 XF11. Additional measurements showed that the asteroid
would not hit Earth in 2028, although it will come close, about 2.5 lunar
distances (954,000 km) away.

A frisson of dread? Yes. A global catastrophe? No.

The asteroid moved away from Earth after 1997 and it has since been
generally forgotten. But the space rock hasn’t really gone away. In fact,
it’s back. 1997 XF11 is gliding by Earth today–on Halloween–for its
closest encounter until 2028.

“On Oct. 31st, 1997 XF11 will pass 25 lunar distances (9.5 million km) from
Earth. It’s closer than it was when it was discovered in 1997,” says Jon
Giorgini, a member of JPL’s Solar System Dynamics Group. There’s nothing to
fear. On the contrary, astronomers welcome the encounter because it is a
good opportunity to study the asteroid up-close.

“We started out knowing very little about this asteroid–only that it’s
approximately 1 km wide. 1997 XF11 has been circling the Sun in total
anonymity for hundreds of millions of years. But now radar is revealing its
nature.”

Giorgini is one of a team of astronomers led by JPL’s Steve Ostro who are
“pinging” the asteroid using NASA’s Goldstone radar in the Mojave desert.

“The first radar measurement obtained last week,” reports Giorgini, “reduced
the uncertainty in the distance to the asteroid by a factor of 540 (from +/-
2210 miles to +/- 4 miles). We can now reliably predict Earth encounters for
an additional 107 years into the future–to the year 2209. There is no risk
of 1997 XF11 hitting Earth during that time.”

The orbit of 1997 XF11 carries it from a point near the orbit of Venus out
to the asteroid belt and back again. One complete trip around the Sun takes
1.73 years. These frequent visits to the inner solar system make 1997 XF11
harder to predict than some other asteroids.

“1997 XF11 has many encounters with Earth and Venus through the years,” says
Giorgini. Gravitational nudges from the two planets perturb the asteroid’s
orbit. “For other asteroids one might obtain several centuries of
predictability from a single radar measurement, but for 1997 XF11 our
knowledge of its position is more quickly ‘blurred.’ The more radar data we
get … the better,” he says.

Giorgini and colleagues will continue observing through November. Radar data
accumulated over a period of time, says Giorgini, can be used not only to
measure orbits but also to create 3-dimensional maps of asteroids. Some have
weird forms: 216 Kleopatra, for example, looks like a dog bone. Learning the
shapes of asteroids helps scientists understand how they’re put
together–valuable information in case we ever need to deflect one or blow
it apart.

Most asteroids (1997 XF11 included) are as dark as charcoal. They shine only
because they reflect a few percent of the sunlight that hits them. When 1997
XF11 is closest to Earth today, it will glow like a 13th magnitude star–too
faint to see with the unaided eye or even binoculars.

Nevertheless amateur astronomers using 10″ telescopes and CCD cameras will
be able to detect 1997 XF11 as it glides through the constellation
Capricornus. Optical observations, notes Giorgini, can tell us a great deal
about the asteroid’s rotation period and mineral composition. In 2028, when
1997 XF11 is only 2.5 lunar distances from Earth, optical telescopes will
play a greater role in its study. The asteroid will brighten to 8th
magnitude, which is within reach of binoculars.

There’s still much to learn. One thing, however, is already clear: we’re
safe from 1997 XF11 for at least another 200 years. This Halloween asteroid
is not so scary after all.